Gas turbines use ambient air as combustion air for the combustion of fuel to generate hot gas, which drives a turbine. A typical industrial gas turbine, which is used as a stationary turbine for the generation of electric power, is shown in FIG. 1. The exemplary gas turbine 10 of FIG. 1 is equipped with sequential combustion. It comprises a compressor 12 for compressing the combustion air, a first combustor 13, a first turbine 14, a second combustor 15 and a second turbine 16. The ambient air to be compressed by the compressor 12 enters the gas turbine through an air intake 11.
FIG. 1 shows the bare gas turbine 10. The ambient air, which is compressed by the compressor 12, usually has to be filtered and in some cases mixed with water by means of a fogging device, before being introduced into the compressor through the air duct, air intake manifold 17, and the air intake 11. In most cases, an air intake manifold 17, as shown in FIG. 2, is used to guide the air from the entrance of a gas turbine plant to the air intake 11 of gas turbine 10.
The air intake manifold 17 of FIG. 2 is made up of several walls 18, which enclose an inner space with an inlet opening 19 and an outlet opening 20, the outlet opening 20 being adapted to the air intake 11 of the gas turbine 10. The air intake manifold 17 has a wall structure 21, which serves several purposes:                it must provide a sufficient mechanical stiffness, which is necessary for a box with outer dimensions of several meters;        it must be airtight so that no unfiltered secondary air enters from outside; and        it should have noise damping insulation properties so that the environment is protected against noise generated by the compressor and the air flowing through the air intake manifold.        
The air intake manifold 17 of FIG. 2 has a wall structure 21 a part of which is shown in FIG. 3. A frame of L beams 22, which is reinforced with crossing ribs 23, supports from the outside an outer sheet 24, which separates the inner space (on the left of outer sheet 24 in FIG. 3) of the air intake manifold 17 from the outside (on the right of outer sheet 24 FIG. 3).
A noise damping insulation is provided on and assembled from the inner side of the outer sheet 24. Insulation ribs 26, made of stainless steel (X6CrNiTi18-10 or similar) are welded on the outer sheet 24 forming square frames (see also FIG. 2). Each frame is filled with a noise damping insulation material 25 made for example of melamine foam with a thickness of 45 mm. The noise damping insulation material 25 or melamine foam is covered with 3 mm thick perforated sheets 27 made of stainless steel. Fixation plates 28 are welded on the insulation ribs to keep the perforated sheets 27 in place. An example of an air intake manifold and wall structure is disclosed in document US 2008/0202848 A1.
This prior art wall structure design has some disadvantages:                there are two support structures: (1) a main structure made of L beams of the dimension 150×150×15 mm reinforced with ribs 100×10 mm from outside, and (2) an insulation structure made of stainless steel ribs 45×4 mm for carrying melamine foam and perforated sheets;        numerous small parts (e.g. fixation plates 28) are welded in the flow channel (inner space of air intake manifold);        deformations of the insulation ribs and walls are possible during the welding process.        